Amazon’s delivery drones could use their propellers to tell people to get out of the way.

A new patent filed by the online retailer describes how propeller speeds could be tweaked to produce sounds which sound like someone saying “watch out”.

The system would be triggered when on-board cameras detect that a person is stood on or near a suitable landing area that the drone would like to use.

Another suggested warning method is for light sources such as on-board LEDs to generate words on the spinning the propellers.

Amazon drones will look for landing spots indicated by customers

Regulators have indicated that they want drones to be as quiet as possible, which is likely why Amazon is exploring these techniques rather than adding speakers to their drones.

The patent also outlines a system for reducing propeller noise overall.

It describes a system where some propellers would be used for life and thrust, while others would be used to cancel out their noise.

The patent application says: “While the second propeller may cause lift of the (drone), it may also be operable to produce sound that cancels noise generated by the first propeller.”

The package would then be released from the craft

Last year Amazon unveiled a new hybrid delivery drone that can travel up to 15 miles at high speed.

In a video one of its drones can be seen landing on a designated spot in someone’s back garden.

A flap underneath the drone opens up, and a package is deposited on the lawn.

Amazon says the drone can fly around obstacles thanks to built-in safety features – an attempt to appease regulators who are generally resistant to the idea of drone delivery.